In the field of arc welding, the main types of welding processes are gas-metal arc welding with solid (GMAW) or metal-cored wires (GMAW-C), gas shielded flux-cored arc welding (FCAW-G), self shielded flux-cored arc welding (FCAW-S), shielded metal arc welding (SMAW) and submerged arc welding (SAW). Of these processes, gas metal arc welding with solid or metal-cored electrodes are increasingly being used for joining or overlaying metallic components. These types of welding processes are becoming increasingly popular because such processes provide increased productivity and versatility. Such increase in productivity and versatility results from the continuous nature of the welding electrodes in gas metal arc welding (GMAW & GMAW-C), which offers substantial productivity gains over shielded metal arc welding (SMAW). Moreover, these electrodes produce very good looking welds with very little slag, thus saving time and expense associated with cleaning welds and disposing of slag, a problem that is often encountered in the other welding processes.
In gas metal arc welding with solid or metal cored electrodes, a shielding gas is used to provide protection for the weld against atmospheric contamination during welding. Solid electrodes are appropriately alloyed with ingredients that, in combination with the shielding gas, provide porosity free welds with the desired appearance and mechanical properties. In metal cored electrodes, these ingredients are on the inside, in the core (fill) of a metallic outer sheath, and provide a similar function as in the case of solid electrodes.
Solid and metal-cored electrodes are designed to provide, under appropriate gas shielding, a solid, substantially porosity free weld with yield strength, tensile strength, ductility and impact strength to perform satisfactorily in the final applications. These electrodes are also designed to minimize the quantity of slag generated during welding; however, small slag islands and/or one or more thin lines of slag at the toes of the weld are often formed during welding. In general, these slag islands are oxides of manganese and silicon that are formed, when these elements that are present in the wire react with oxygen during welding. After welding, these slag islands or slag lines are removed to provide a clean surface that, if desired, can be later treated (e.g. painted or coated) to enhance appearance, inhibit corrosion, etc. Failure to remove the slag can result in peeling of the slag after the weld has been painted or coated, which can result in corrosion at that site or negatively impact the cosmetic appearance of the weld.
Metal-cored electrodes are used increasingly as an alternative to solid wires because of increased productivity during welding fabrication of structural components. Metal cored electrodes are composite electrodes consisting of a core (fill) material surrounded by a metallic outer sheath. The core consists mainly of iron powder and alloying and fluxing ingredients to help with arc stability, weld wetting and appearance etc., such that the desired appearance and mechanical properties are obtained in the weld. Metal cored electrodes are manufactured by mixing up the ingredients of the core material and depositing them inside a formed strip, and then closing and drawing the strip to the final diameter. Metal-cored electrodes provide increased deposition rates and produce a wider, more consistent weld penetration profile compared to solid electrodes. Moreover, they provide improved arc action, generate less fume and spatter, and provide weld deposits with better wetting compared to solid electrodes. However, these productivity improvements are sometimes offset by the expense incurred because of the time required to remove the slag deposits or islands, which form on the surface of the weld.
In general, in gas metal arc welding with solid or metal cored wires, the slag islands tend to form at the toes of the weld. The slag islands get wedged in the toes and this makes them very difficult to remove. In this invention, addition of ingredients to the core (fill) of the metal core electrode have been made, which allow for the slag to form as discrete islands in the middle of the weld, instead of the toes of the weld. This allows the slag islands to either self detach or be removed easily.
Several fill compositions have been developed to address the slag removal problem. In U.S. Pat. No. 4,345,140 to Godai, a flux composition use in a cored electrode for welding stainless steel is disclosed. Godai discloses that the addition of low melting point metallic oxides such as lead oxide, copper oxide, bismuth oxide, antimony oxide or tin oxide is useful in enhancing the separability of slag. The teachings of Godai are incorporated herein by reference.
Another fill composition having improved slag removal is disclosed in U.S. Pat. No. 6,608,284 to Nikodym. Nikodym discloses a fill composition for a mild steel or low alloy steel electrode. Nikodym distinguishes the disclosed fill composition from the fill composition disclosed in Godai on the basis that Godai is directed to a flux cored electrode for stainless steel welding which is fundamentally different from metal-cored electrodes for mild steel and low alloy steel welding. Nikodym asserts that flux cored electrodes for use in the welding of stainless steel include a flux composition consisting of nonmetallic inorganic components that are present in significantly higher percentages (e.g., 5 to 10%) than in metal cored electrodes for use in the welding of mild or low alloy metals, thus resulting in the slag covering the entire surface of and adhering strongly to the weld bead thereby making it very difficult to remove. The fill composition disclosed in Nikodym includes the addition of antimony, bismuth and/or germanium to a weld metal to cause slag deposits or islands on the weld metal to form at positions away from the toe or edge of mild and low alloy steel weld beads, thereby facilitating the removal of the slag deposits or islands. The teachings of Nikodym are incorporated herein by reference.